DESCRIPTION (Applicant's abstract): The phospholipase-mediated deacylation of phosphatidylinositol (P1) to produce extracellular glycerophosphoinositol (GroPIns) is a major catabolic pathway in S. cerevisiae cells grown in inositolcontaining medium. In times of inositol limitation, GroPlns is transported back into the cell in an energy requiring process mediated by the product of the GIT1 gene. This novel membrane permease has a number of interesting features that make it worthy of intensive study. Gitip is the first GroPlns transporter to be characterized in any organism, and there is evidence for the existence of such a transporter in higher eukaryotes. The GroPlns transported through Gitlp is used for the synthesis of PT and its inositol-containing derivatives, many of which play important roles in signal transduction. Thus, the activity of Gitip may impinge upon a number of important cellular events. In addition, evidence suggesting that the transport of GroPlns through Gitip is regulated by both phospholipid metabolism and phosphate metabolism raises the prospect of characterizing the regulatory networks linking these two fundamental aspects of cellular metabolism by means of the study of Gitip. Finally, the unique chromosomal location of GIT1 provides an opportunity to study the role of transcriptional silencing in regulating GIT1 expression. The Specific Aims of this proposal are to examine the regulation of GroPlns transport through Gitip at the transcriptional and posttranslational levels by 1) phospholipid metabolism, 2) the Unfolded Protein Response Pathway, 3) phosphate metabolism ,and 4) gene location.